linux-kernel - Re: [PATCH v3 13/29] riscv mmu: write protect and shadow stack

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Date: Sun, 12 May 2024 18:31:24 +0200
From: Alexandre Ghiti <alex@...ti.fr>
To: Deepak Gupta <debug@...osinc.com>, paul.walmsley@...ive.com,
 rick.p.edgecombe@...el.com, broonie@...nel.org, Szabolcs.Nagy@....com,
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Subject: Re: [PATCH v3 13/29] riscv mmu: write protect and shadow stack

On 04/04/2024 01:35, Deepak Gupta wrote:
> `fork` implements copy on write (COW) by making pages readonly in child
> and parent both.
>
> ptep_set_wrprotect and pte_wrprotect clears _PAGE_WRITE in PTE.
> Assumption is that page is readable and on fault copy on write happens.
>
> To implement COW on such pages,


I guess you mean "shadow stack pages" here.


>   clearing up W bit makes them XWR = 000.
> This will result in wrong PTE setting which says no perms but V=1 and PFN
> field pointing to final page. Instead desired behavior is to turn it into
> a readable page, take an access (load/store) fault on sspush/sspop
> (shadow stack) and then perform COW on such pages.
> This way regular reads
> would still be allowed and not lead to COW maintaining current behavior
> of COW on non-shadow stack but writeable memory.
>
> On the other hand it doesn't interfere with existing COW for read-write
> memory. Assumption is always that _PAGE_READ must have been set and thus
> setting _PAGE_READ is harmless.
>
> Signed-off-by: Deepak Gupta <debug@...osinc.com>
> ---
>   arch/riscv/include/asm/pgtable.h | 12 ++++++++++--
>   1 file changed, 10 insertions(+), 2 deletions(-)
>
> diff --git a/arch/riscv/include/asm/pgtable.h b/arch/riscv/include/asm/pgtable.h
> index 9b837239d3e8..7a1c2a98d272 100644
> --- a/arch/riscv/include/asm/pgtable.h
> +++ b/arch/riscv/include/asm/pgtable.h
> @@ -398,7 +398,7 @@ static inline int pte_special(pte_t pte)
>   
>   static inline pte_t pte_wrprotect(pte_t pte)
>   {
> -	return __pte(pte_val(pte) & ~(_PAGE_WRITE));
> +	return __pte((pte_val(pte) & ~(_PAGE_WRITE)) | (_PAGE_READ));
>   }
>   
>   /* static inline pte_t pte_mkread(pte_t pte) */
> @@ -581,7 +581,15 @@ static inline pte_t ptep_get_and_clear(struct mm_struct *mm,
>   static inline void ptep_set_wrprotect(struct mm_struct *mm,
>   				      unsigned long address, pte_t *ptep)
>   {
> -	atomic_long_and(~(unsigned long)_PAGE_WRITE, (atomic_long_t *)ptep);
> +	volatile pte_t read_pte = *ptep;
> +	/*
> +	 * ptep_set_wrprotect can be called for shadow stack ranges too.
> +	 * shadow stack memory is XWR = 010 and thus clearing _PAGE_WRITE will lead to
> +	 * encoding 000b which is wrong encoding with V = 1. This should lead to page fault
> +	 * but we dont want this wrong configuration to be set in page tables.
> +	 */
> +	atomic_long_set((atomic_long_t *)ptep,
> +			((pte_val(read_pte) & ~(unsigned long)_PAGE_WRITE) | _PAGE_READ));
>   }
>   
>   #define __HAVE_ARCH_PTEP_CLEAR_YOUNG_FLUSH


Doesn't making the shadow stack page readable allow "normal" loads to 
access the page? If it does, isn't that an issue (security-wise)?